1 Introduction
During the past ten years, multiple national and European funding programs adressed the resource potential of mining waste (coarse tailings, waste rockpiles, and smelter slag) with a focus on the exploitation of new sources for critical raw materials that were defined as particularly important for the European high-tech industry by the European Committee (1). They are based on the European and national resource strategy (2). One of these programs in Germany was “r3 - Strategic Metals and Minerals – Innovative Technology for Resource Efficiency”, which was founded in 2012 and funded by the Federal Ministry of Education and Research (BMBF). Its aim was to ensure the supply of the German economy with strategically significant metals and minerals, and to support projects in the fields of recycling, substitution and reduced resource usage, urban mining, and methods for the evaluation of resource efficiency.
As part of this program, the Helmholtz Institute Freiberg for Resource Technology (HIF) collaborated with partners such as the Technische Universität Bergakademie Freiberg (TUBAF) on various projects on the characterization of mining wastes, and resource exploitation.
HIF pursues the objective of developing innovative technologies for the economy to increase the efficiency of access, use, and environmentally responsible disposal of mineral and metalliferous resources. The HIF was founded in 2011 by the German government as part of the national resource strategy. It is part of the Helmholtz Zentrum Dresden-Rossendorf (HZDR) and is closely tied to the TUBAF. In addition, the HIF is an important member of the European network EIT RawMaterials and played a significant role in its establishment.
However, the potential of critical metals in mining waste is not the only topic of current political and public interest. After the catastrophic dam failures in Brazilian mining heaps, e. g., in the Corrego do Feijão mine in Brumadinho (VALE) in 2019, public pressure on the mining industry, tailing operators and managers, e. g., states, and politics increased to lower such risks. The Global Standard on Tailings Management was a new policy developed to prevent such accidents in the future. The International Council on Mining and Metals (ICMM), the UN environment programme and the Principles for Responsible Investment (PRI) collectively advocate for the implementation of global best practices for mining waste. Together they appealed for the global examination of waste dumps to determine an international standard (3).
Due to their environmental risks as well as their high potential as a source for critical and valuable metals, projects on mining waste are very complex. There is a global need for new methods and solutions that are developed from different perspectives with holistic and sustainable approaches under consideration of ecological, technical, social and economic aspects. For this reason, HIF, together with TUBAF, has founded the recomine alliance and coordinates it. The predominantly regional network (Ore Mountains) is developing innovative and holistic solutions with their existing know how and is funded by the BMBF as part of the WIR! program.
2 Major research projects on contaminated soils at the HIF
As part of the r3-program, the HIF began working on the issue of mining waste in the project “Strategic Metals and Minerals from Mine Waste in Saxony (SMSB)”. In collaboration with their partners, G.E.O.S Freiberg GmbH, TUBAF, AKW Apparate + Verfahren, and SAXONIA Standortentwicklungs- und verwaltungsgesellschaft mbH, the experts investigated how exactly the mineralogy of old waste dumps can be characterized and how critical raw materials (according to the definition of Critical Raw Materials – CRM by the European Committee) can be extracted in an efficient and environmentally friendly way.
Within the scope of the project, the HIF used the term “re-mining” to describe the sustainable recycling of mining waste and the holistic “near zero waste” approach. The term represents the idea of minimizing the environmental risk of waste dumps that contain large amounts of heavy metals and simultaneously extracting valuable metals. Within the SMSB project, data of the 20 biggest mine waste dumps and tailings in Saxony – including their geographical position, ownership, structure, raw material content, value-added potential of the heaps as well as the origins of the contained materials – were registered in a data base. Two of the waste dumps were examined in detail via a drilling campaign and sampling in combination with processing tests of the overburden material by using flotation, and bioleaching technology. Based on the detailed mineralogical data from the liner samples, e. g., particle size, geochemical information (concentration of valuable metals/minerals) and the liberation of minerals that were measured with an automated mineralogical analysis, a 3D tailingsmodel was constructed (Figure 1) in which geostatistical interpolation methods and the integration of old data were used.
Due to a weighting function for the three most important material characteristics for preparation the model considers particle behaviour in the processing line and delivers a realistic estimation of the resource extraction in the examined pile (4).
In the interdisciplinary SMSB project HIF has further developed its know-how in several disciplines, e. g., remote sensing, mineralogical characterisation of mining waste, geostatistical 3D modelling, bio-leaching of sulfidic tailings and the fine particle processing by flotation. The promising results of the project have high potential for appliance in the raw material sector, however, there is still room for improvement. Over the past years, knowledge about the characterization and processing of mining waste has been broadened in many follow-up projects.
In these projects, partly completed, partly on-going, many valuable insights regarding the realistic estimation of resource potential of heaps have been gained. The most important lesson that can be learned from the mining waste projects of the HIF, is that there is no general method for the estimation of mine waste potential. Each mine waste dump is different. Each site has its own history. In the past, different technologies were used for different primary resources in a particular regional area. Even if there is a high content of valuable metals and minerals, there is no guarantee for the successful reprocessing of heaps. Besides, technical challenges, that mainly depend on the mineralogical parameters of the material, the success in other challenges decides whether the mineralogical potential can be transferred into an efficient and ecological reprocessing project. Three categories of characteristics are to be considered: the mineralogical and site-specific potential, the technological potential, as well as social and ecological aspects.
2.1 Mineralogical and site-specific potential
At the beginning of each investigation of mining waste, it is necessary to accumulate all available publications, data, maps and older information about past mining and processing activities and the tailing itself. It is of great importance to know the past processing flowsheet and to understand which part of this process the waste dumps resulted from. There is a significant difference between material from a shaking table and material from other technologies such as magnetic separators or flotation cells. This information about past processing can give an impression of the current state of the material.
The mineralogical potential of the mining waste must be characterized by detailed mineralogical and geochemical analyses. It is necessary to understand the mineralogical composition of the material, the liberation and particle size distribution, to decide which type of processing leads to efficient extraction of the desired minerals and metals. Moreover, further important questions regarding content of toxic elements and minerals within the material will be answered. In accordance with national laws and regulations, the hazardous components of the mining wastes must be subjected to a special, costly disposal or treatment if the concentration exceeds certain levels which must be considered in a feasibility study for re-mining.
Other site-specific parameters that are part of this category must be investigated as well. The volume and homogeneity of the investigated site are two of those parameters, since both are needed to estimate the actual amount of valuable metals in the heap. The volume defines a specific body in which valuable metals are to be expected. It can be taken from old data or determined via remote sensing technology (Figure 2) in combination with old maps.
Informations of homogeneity are important for knowing whether material parameters and the concentration of precious metals within a heap are changing significantly. Thus, a geostatistical sample-taking strategy is necessary to obtain representative samples with reliable information about homogeneity and mineral characterization. Mine waste dumps also have great value in regard to the covered area. If the site is located near larger cities, the value of the emerging property can be considered in a feasibility study if the amount of material is considerably reduced by reprocessing. Metal prices also have an impact on the efficient feasibility of a reprocessing project. In case of volatile metal prices, it is important to calculate different scenarios. Finally, it is important to closely analyse the site-specific infrastructure. There is a difference between an area with on-going mining and processing activity with active facilities and an abandoned site in need of capital-intensive investments in new facilities. It is necessary to know the distance to the closest smelting operation which can be a potential purchaser of locally generated metal concentrates. Based on this distance, transport costs can be estimated.
2.2 Technological potential
Based on a detailed investigation of the mineralogical and site-specific potential, the technological potential must be evaluated. The aim must be to answer questions on whether a technology is capable of efficiently extracting the desired metals and minerals from the material. Good understanding of the target minerals’ parameters, and the material matrix is required to choose a suitable technology. This means a site-specific combination of different technologies, such as milling, crushing, magnetic separation, chemical and biological leaching, flotation, hydrometallurgical and pyrometallurgical steps, sensor-based sorting, density separation and more. Pilot scale experiments must test and optimize the performance of each step in a flowsheet. Some of the most important flowsheet parameters are
- total energy consumption of the facility;
- recovery rate of processing activities;
- yield in the final concentrate;
- percentage of toxic elements in the concentrate;
- facility performance;
- logistic costs of the operation for concentrate production.
All these parameters have significant impact on CAPEX, OPEX and revenue. Each individual parameter can lead to a reprocessing activity becoming inefficient and must therefore be considered. However, economic use must also be incorporated in the resulting concepts since holistic recycling of heaps can save the government, or mining operation, more money long-term than traditional rehabilitation (coverage with on-going control/maintenance) that causes eternity costs.
2.3 Social and ecological aspects
Aside from the technical, mineralogical and site-specific aspects, there are always certain non-technical, social aspects in a working area that can affect reprocessing projects positively or negatively. These aspects must be investigated. They can be distinguished in two categories: environmental aspects and social aspects. Environmental aspects can be viewed from two perspectives. From one perspective, mining waste is seen as a source of emissions that can cause negative mutual reactions in the environment. Contamination caused by heavy metals, contaminated dust emissions or acidic drainage can impact the surrounding ecosystems negatively. Usually, this results in pressure on local authorities to take rehabilitation measures that are very expensive for the entire community.
If a holistic re-mining concept can eliminate or immobilize these hazardous emissions, while simultaneously winning valuable metals, the community can profit from this new, sustainable rehabilitation concept. Earnings from the metal sale can refinance the rehabilitation so that less public money must be used for the emission issue. In addition, no eternity costs are created in comparison to traditional rehabilitation, which usually covers the waste with a dense layer, since hazardous compounds are extracted, concentrated and disposed in small amounts in special dumps, or steadily immobilized during the process. Moreover, a new source of critical resources is created, which can assist the supply of high-tech industrial nations that depend on CRM.
However, environmental aspects can also interfere with re-mining projects. Abandoned mining dumps have often been deserted after mining activities had been shut down. Nature has often reclaimed these areas where rare, heavy metal resistant plants could grow unhindered and rare animals could generate new populations. Since such habitats are scarce in our man-made landscape, these abandoned mining dumps are often refugia for highly protected and rare species. European and national environmental protection laws are very strict, e. g., regarding the sand lizard in Germany. Processing mining waste would result in disturbing these protected animals, which is legally possible, if there is a legitimate collective interest, i. e. a high risk of toxic emissions. The motivation of economic metal winning, even if it supplies critical metals to the national industry, is legally insufficient. This aspect is very important and can cease re-mining activity. Figure 3 shows how nature has progressed in the Davidschacht tailing in Freiberg that was shut down over 40 years ago.
The category of social aspects is closely linked to local history. Land ownership is one of these aspects. While mining was run by state-owned operations in the former German Democratic Republic (GDR), the working area was often dispossessed property that had been merged by authorities to one or multiple larger parcels of land. Nowadays, e. g., in the newly formed German States most mining wastes are located on a few or one individual parcel that usually belongs to a single owner.
In the former West German States mining wastes are often spread over more than ten parcels that all belong to different owners. To initiate a reprocessing project, all owners would have to sell their parcels, or at least be willing to participate in such a project, which can hinder the realisation of a reprocessing enterprise. This is mainly a German issue. However, it must be considered to estimate the actual re-mining potential in this country.
In 2020, the mining region Ore Mountains was named a World Heritage Site. This resulted in another social aspect of importance. Historical mining sites including heaps with high mineralogical potential are part of the protected area. To re-work these areas, it is necessary for the UNESCO-department in Paris to first approve the concept to prevent the region from losing its World Heritage status. Should the re-mining enterprise jeopardize this status, it is very likely that it will not be collectively accepted which leads to the abortion of the project.
Actual land use, or classification is another important criterion. Heaps can be overbuilt with civic infrastructure, such as solar plants, or be part of designated woodland or preserved areas. The accessibility of these mine waste dumps must be evaluated before the start of a project. If the site is located in a national park or in a preserved area, it is unlikely that a reprocessing project will be licensed.
After all, social acceptance is generally of great importance. How is the project going to impact surrounding communities? Is mining part of the regional identity, or would re-mining interfere with this identity, e. g., if it is a tourist region and the dust and noise pollution caused by trucks impacts businesses negatively.
3 recomine – conceptual design for resource-oriented environmental technology
It is important for the future to develop new holistic concepts for mining waste that not only consider social, but also technical and mineralogical aspects and can combine resource technology, environmental technology and digitalization to create social values and sustainable solutions for modern mining.
For this reason, the HIF founded the recomine alliance in 2018 that is funded by the BMBF as part of its WIR! programme until 2025 and will act independently from then on. The association shares a vision of further developing local skills in the field of environmental, and resource technology for the exploitation of disperse raw material sources. Common contaminated sites such as the trial and pilot areas in the Ore Mountains are examples for mining waste sites around the globe. The rehabilitation of these sites can be financed by reprocessing leftover raw materials. Based on numerous specific activities in this field, e. g., equity-financed research programmes and industry competitions, such as the BHP Tailings Challenge, a global increase in the need for holistic solutions for contaminated sites in the mining industry can be seen. Since the publication of the new “Global Industry Standard on Tailings Management” 2020 in London, that focuses on its “zero harm” concept (no more harm caused by contaminated sites), many mining companies have worked out concrete measures for the reduction and recycling their mine waste. This holds high potential for further development and the global marketing of this know-how for the Ore Mountains region, that is very experienced in the handling of contaminated sites and for the recomine association itself. The recomine vision is shown in figure 4.
Typical recomine projects adress coarse mine waste and tailings, pit and mine water, as well as slags and ashes since these material flows are considered the main contaminated waste streams from the mining industry. In addition, social issues that are closely connected to the topic will be addressed.
Local industry partners and research institutions not only have comprehensive know-how in resource and environmental technology, but also considerable knowledge of automation and sensor technology. By using this know-how, future-proof and highly efficient technology can be established.
The mining and smelter history of the Ore Mountains region, that dates back multiple centuries, makes the region an ideal model location for the development of environmental and resource-oriented technology in pilot facilities for the international market. For this reason, five development sites were set up in the recomine alliance:
- the Davidschacht tailing in Freiberg;
- the Rote Graben in Tuttendorf (below (1));
- the tailings I & II and the drainage adit in Ehrenfriedersdorf;
- the historic smeltery Muldenhütten; and
- the IAA Bielatal in Altenberg.
In the future, more locations, e. g., in Bad Schlema (in collaboration with the Wismut GmbH) and Freiberg (new metallurgy technical centre of the HIF) are to be used for technology development, training and research. Figure 5 shows, which subject matters are of importance in the recomine development sites and can be addressed in projects.
By combining comprehensive know-how in recomine, the association successfully contributed to the BHP Tailings Challenge with a concept for the recycling of copper mine heaps. With their modular concept, recomine managed to get into the top 10 of 153 contending teams. Their modular approach is now being tested in the on-going proof-of-concept phase. In the beginning of 2022, it will be decided whether the alliance will set up one of the three pilot facilities in the subsequent phase (6). Further information on the association, current projects and the BHP Tailings Challenge can be found under www.recomine.de.
References/Quellenverzeichnis
References/Quellenverzeichnis
(1) European Commission (2020): Critical Raw Materials Resilience: Charting a Path towards greater Security and Sustainability. Communication from the Commission to the European Parliament, the Council, the European Economic and Social Committee and the Committee of the Regions. https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:52020DC0474&from=EN
(2) Rohstoffstrategie der Bundesregierung. Bundesministerium für Wirtschaft und Energie, 2010.
(3) Website: https://globaltailingsreview.org/, 29.09.2020.
(4) Büttner, P.; Osbahr, I.; Zimmermann, R.; Leißner, T.; Satge, L.; Gutzmer, J. (2018): Recovery potential of flotation tailings assessed by spatial modelling of automated mineralogy data. In: Minerals Engineering, Vol. 116, pp. 143 – 151, ISSN 0892-6875.
(5) Eigene Darstellung nach einer durch das Helmholtz-Institut Freiberg beauftragten naturschutzfachlichen Biotopkartierung (2017) der Spülhalde Davidschacht durch das Naturschutzinstitut Freiberg und Abbildungen der SAXONIA Standortverwaltungs- und -entwicklungsgesellschaft mbH.
(6) https://expandemineria.cl/convocatoria-custom/bhp-tailings-challenge/?lang=en